According to APNG specs, the first frame can be marked hidden.
This test:
Frame 0: green (hidden)
Frame 1: red
This test assumes canvas drawImage() is sophisticated enough be able to return hidden apng frame (green).
Instead the image decoders in both Firefox and WebKit prefer not to keep the useless hidden frame in memory, so both return the first *visible* frame.
You can see Firefox draws red frame too:
http://w3c-test.org/2dcontext/drawing-images-to-the-canvas/2d.drawImage.animated.poster.html
So I think we should simply mark this test accordingly.